The present invention relates to a process for determining a nominal value equipment curve of an installation for controlling the capacity of a pump actuated by an electric motor with speed control, by means of which pump a fluid is transported in said installation via predetermined conduits and consumers in an installation, the number and inside cross-sectional area of which are variable, in which controlling process a physical variable representative for the capacity of the pump is detected and an electrical signal proportional to this physical value is input in a controller as an actual value, the controller controlling the rotational speed of the electric motor and thus the pump capacity on the basis of the curve of nominal values of the installation.
Pumps actuated by an electric motor, in particular rotary or centrifugal pumps, are frequently used in installations wherein the pump capacity demanded by the installation changes with time. This is for instance the case with pumps in heating installations. Here, maximum capacity is only required if all consumers are connected. In practical operation, however, the consumers, for instance individual heaters, are operated at reduced level only or are completely disconnected at certain times, which may result in completely different operating conditions with different capacity requirements variable with time. However, the maximum capacity of the pump in such an installation always has to be set so that all consumers within the installation can be supplied sufficiently even if all of them are connected at maximum consumption at the same time. If the pump is operated at full capacity all the time, i.e. also at reduced capacity requirements, the capacity of the machine willxe2x80x94unnecessarilyxe2x80x94be raised beyond the level required. This behavior is illustrated with reference to a rotary fluid pump in the upper part of the graph of FIG. 1, showing a family of characteristic curves of the rotary pump, i.e. the pumping head H as a function of the capacity Q at a certain rotational speed n. This graph only shows the characteristic curve of the pump for its rotational speed of n=50 Hz in its entirety, other characteristic curves of the pump for n=40.5 Hz, n=32.4 Hz, n=26.3 Hz, and n=23.2 Hz are only shown as curve portions.
Dimensioning of the required capacity of such a rotary pump in an installation results from setting an operation value Bmax, which is the point of intersection of a demanded maximum volume pumped Qmax (in this example 80 m3/h), when all consumers are fully connected, and the pumping head H required for smooth operation of the installation (in this example 13 m). The characteristic pump curve intersecting this operating point Bmax (in this example the curve for n=50 Hz) gives the maximum rotational speed of the pump necessary for maintaining the required pumping head at 100% consumption (=Qmax). But if the actual consumption within the installation decreases and the pump continues to be operated at the same rotational speed n=50 Hz, the pumping head H rises, i.e. the instantaneous point of operation rises toward the left along the characteristic pump curve. Thus it can be seen that operating an installation in this way is highly uneconomical. In addition there may be disturbing noise resulting from flow if the output is considerably higher than that actually needed in the installation.
In order to improve a pump""s energy consumption and its noise level, it is known to control the pump output (=pumping headxc3x97volume pumped) of a rotary pump in such a way that a constant pumping head results at the pump outlet regardless of the respective volume pumped, which pumping head may be measured by means of a pressure sensor and fed to a controller as an actual value.
However, controlling at a constant pumping head in such a way does not take into consideration the actual conditions within the installation. In particular, it does not take into consideration the inevitable pressure drops within the circuit system and the consumers connected thereto.
In order to improve control of the pump of such an installation it is thus necessary to define a curve of nominal values for the installation, taking into consideration the installation losses as a function of the volume pumped at least to a certain extent.
The controlling behavior of such an installation with a variable curve of nominal values therefor is shown in the graph of FIG. 1, once as the curve of nominal values for the installation Hinstallation(Q) in the graph of pumping head versus amount pumped, and again as the curve of nominal values for the installation Pinstallation(Q) in the graph of power uptake versus amount pumped. Both curves of nominal values for the installation are approximately parabolic in shape, but they have different slopes.
The graph of power uptake versus amount pumped also includes the respective energy savings with a pump control employing the given characteristic curve of nominal values for the installation as compared to operation of the pump at a constant rotational speed of n=50 Hz.
In practical operation, however, there is the problem of how to obtain an appropriate characteristic curve of nominal values for the installation. It is known to establish such curves of nominal values for an installation in tabular form by means of conduction and consumer ratings, or to define loss curves as mathematical functions. Both processes are theoretical approaches with which it is not possible to take into consideration the actual conditions within the installation, for instance a reduction of the cross-section of the conduit due to clogging or calcification, leaks of the installation, eddying because of bends or pronounced turns of the conduits. Thus many pumping systems also offer an operator the possibility to manually input tables of nominal values in a controller, which values have been fixed on the basis of experience or previous measurements of the installation (see e.g. DE-OS 37 04 756). It can easily be seen, however, that in all the above processes, the conditions within the installation are not taken into consideration to a sufficient extent.
European Patent No. 0,444,269 discloses regulating the power output of a pump which is driven by a speed-regulated electric motor in a closed system comprising conduits and consumers by first, during a calibration process for some arbitrary initial state of the system, recording, at least step-wise, the electrical signal from a probe detecting the rate of fluid flow at a plurality of points using particular settings of the pumping power and the appertaining percentage rates of flow of the fluid, from the static state of the pump up to 100% power output thereof, which corresponds to a 100% rate of flow, and supplying this respective signal as measurement value to a computer which produces a characteristic curve passing through each of the test values delivered by the probe, regulation of the output of the pump by the computer initially being in correspondence with this characteristic curve as long as it is within the limits set by the calibration process, until such time as, due to changes in the system, a greater rate of flow of the fluid occurs than for a 100% power output of the pump during the calibration process, whereupon the computer defines the highest value of the rate of flow as a new test value for a 100% power output of the pump on an extension of the characteristic curve, and the computer displaces each of the percentage values of the rate of flow along the characteristic curve by an amount which corresponds proportionately to the ratio of the new highest rate of flow to the preceding one, while an adaptation of the characteristic curve is effected in a manner analogous thereto by the computer whenever a further increase in the rate of flow occurs.
The present invention provides a process for determining a curve of nominal values for an installation with which the above disadvantages of the prior art will be avoided and which does justice to the actual conditions of the installation.
The process according to the invention comprises the following steps:
a) closing all consumers in the installation,
b) detecting a consumer operation parameter at a consumer at a distance from the pump, preferably at the consumer positioned at the greatest distance from the pump, which parameter is representative for the operability of this consumer, and varying the instantaneous pump capacity until the consumer operation parameter has reached a predetermined value,
c) transmitting a confirmation signal to the controller as soon as the consumer operation parameter has reached the predetermined value,
d) upon receipt of the confirmation signal, determining the instantaneous value of a pair of pump parameters representative for the instantaneous capacity of the pump, and storing this value in the controller,
e) opening one or several consumers and repeating the above process steps b) through d), and
f) calculating a function by way of a mathematical process for establishing a curve from the stored values of the pump parameter pairs, and storing this function as curve of nominal values for the installation in the controller.
The curve thus calculated does not necessarily have to pass through all parameter pairs determined, but for an optimized curve path may also run therebetween.
Preferably the consumer operation parameter is determined at the consumer at the greatest distance from the pump as it may be assumed that the value of this parameter is minimal for the last consumer because of the losses within the circuits, so that if the adjustment is correct for the consumer at the greatest distance, all other consumers in the system will be supplied to a sufficient extent as well. If, however, the interrelationship between the values of the consumer operation parameter for the consumer at the greatest distance and the values for another consumer is known from empirical experiments or calculations, this other consumer may be used for carrying out the process as well, the known interrelationship having to be considered accordingly.
Transmission of a confirmation signal to the controller as soon as the consumer operation parameter has reached the predetermined value can preferably take place in automated fashion, for instance via remote data transmission. In a simple variant the transmission of the confirmation signal may, however, also be done manually, for example by contact between two operators, one of whom has access to the controller, while the other one has his post at the remote consumer.
In order to achieve maximum precision of the curve of nominal values for the installation process step e) is advantageously carried out several times, but in any case at least twice.
Preferably the consumer operation parameter is a pressure differential between the inlet and the outlet of the consumer at the greatest distance, as such a pressure differential may easily be determined by temporarily connecting a pressure differential sensor to the inlet and the outlet of the consumer.
Preferably the pair of pump parameters comprises the amount pumped and the electrical power taken up by the pump driving motor, and is detected by measuring two parameters from among the amount pumped, the electrical power taken up by the pump driving motor and the rotational speed of the pump, as the case may be optionally by calculating the missing parameter from the mathematical interrelationships between these three parameters. For it is possible to give a family of curves representing a characteristic progression of the uptake of electrical power by the pump driving motor for any rotational speed of the pump as a function of the volume pumped. As it is possible to integrate the measurement of the rotational speed and of the power uptake into a pump control apparatus or a frequency converter, it is thus possible to obtain a pump controlling apparatus the construction of which is superior in its operational reliability and cost-effectiveness.
An alternative preferred embodiment of the process according to the invention is characterized in that the pair of pump parameters comprises the volume pumped and the pumping head and is determined by measuring two parameters from among the amount pumped, the pumping head and the rotational speed of the pump, optionally by calculating the missing parameter from the mathematical interrelationships between these three parameters. For it is also possible to give a family of curves representing a characteristic course of the pumping head as a function of the volume pumped. As the measurement of the rotational speed may be integrated into a pump controller or a frequency converter, and as the pumping head may be determined by means of a pressure differential sensor between pump outlet and pump inlet, this embodiment enables the provision of a pump controller the construction of which is of superior operational reliability.